This invention relates to a power supply system for controlling the output power in the form of the intensity of electromagnetic radiation from an electric discharge lamp and, more particularly, is related to such a system that provides to the lamp AC electric power at suitable levels to avoid extinction of the lamp while controlling the output power over a wide range. Moreover, the invention is directed to such a power supply system and lamp used in conjunction with a conveyor printing press, or the like, with feedback signals being provided between the power supply system and the press equipment to interrelate the same for effective curing of printed material.
Ultraviolet and infrared electromagnetic radiation may be used to expedite the curing of certain inks or paints on surfaces of paper, metal, wood, plastic and the like. In the past conventional ballast circuits have been used to energize an electric discharge lamp at full power and at 70% power for such curing purposes with a large mechanical apparatus being required for any further attenuation of radiation short of lamp extinction. Such prior art ballast circuits vary both the lamp voltage and current with external ambient conditions. Design parameters of the ballast do not take into consideration the many variables which may occur in normal operation. The result of this internal problem will provide unstable operation of the lamp. Once the lamp is extinguished, time must be taken to allow the mercury to condense, then to reapply power to assume normal operation. Up to 8-10 minutes may be lost.
In an electric discharge lamp for curing ink or paint, it is important that a wide range of control of output power be available. For example, if a press were to slow down, it would be necessary to reduce the lamp output intensity, else the printed material may be burned. Similarly, if the press were to stop briefly, it is important that the lamp output be reduced to a minimum short of extinction, first, to avoid burning the printed material or the press web and, second, to avoid the need for a re-start and warm up period after the press is ready to begin again. Moreover, if the curing is not effective the lamp output intensity should be increased and if the called for intensity is not attainable, then the press or other conveyorized mechanic should be slowed. The conventional energization circuits for electric discharge lamps do not provide for such variations and controls, and since the conventional method of light attenuation is achieved mechanically, large space and heat dissipation requirements are necessary and a great deal of electric energy is wasted.
The instant invention will be described hereinbelow with reference to a variable AC power supply system for a mercury vapor electric discharge lamp that emits, upon energization, electromagnetic radiation at least in one or both of the infrared and ultraviolet spectral ranges that is useful for the curing of ink or paint on a substrate material. It is to be understood, however, that the variable AC power supply system may be used to control the power supplied to other types of electric discharge lamps to effect adjustable output power therefrom over a relatively wide range of, for example, from 5 to 100% of maximum output power.
In a mercury vapor electric discharge lamp, which usually comprises a sealed envelope having two interior electrodes an inert gas; e.g. argon, neon and a quantity of fluid mercury filling in liquid and/or vapor form, a high starting voltage applied across the electrode ionizes the inert gas within the tube. The heat developed by this plasma vaporizes the mercury. Steady state conduction of current between the electrodes and through the envelope will occur due to the thermal ionization of the mercury gas or vapor, with temperatures in excess of 3,600.degree. K. being generated in the plasma resulting in a large radial flow of thermal energy.
The mercury vapor electric discharge lamps have a negative resistance characteristic. At start-up a relatively high voltage is required to effect current flow between electrodes through a correspondingly high impedance of the molecules and ions therebetween. After the lamp has been started and voltage to the electrodes is briefly interrupted, a certain number of ions will remain within the envelope to effect conductions. Assuming voltage is re-applied before the extinction time has elasped, this voltage will create a field within the envelope sufficiently strong to sweep any thermionically emitted electrons through the gas, and continued application of such a voltage will re-establish the plasma arc through the envelope. After starting, the hottest gas would be toward the center of the lamp tube due to the radial flow of thermal energy, and this area would contain the largest number of positive ions, which would then be the best conductor with the corresponding result that the current density in this area would be the greatest within the tube. The current density would then continue to increase with a corresponding increase in temperature, which will generate more ions which will then further increase the conduction, the net effect being that less electrical energy, or voltage, is required to push the same number of electrons per unit time through the mercury arc as the total number of electrons per unit time is increased. This phenomena then is apparent as the negative resistance characteristic of the plasma arc are dynamic and are directly associated with the thermal and ionic equilibriums within the envelope.